Improve 2q block collection via 1q quaternion-based collection

crates/accelerate/src/convert_2q_block_matrix.rs

@@ -15,19 +15,18 @@ use pyo3::prelude::*;
 use pyo3::Python;
 
 use num_complex::Complex64;
-use numpy::ndarray::linalg::kron;
-use numpy::ndarray::{aview2, Array2, ArrayView2};
+use numpy::ndarray::{arr2, aview2, Array2, ArrayView2, ArrayViewMut2};
 use numpy::PyReadonlyArray2;
 use rustworkx_core::petgraph::stable_graph::NodeIndex;
 
 use qiskit_circuit::dag_circuit::DAGCircuit;
-use qiskit_circuit::gate_matrix::ONE_QUBIT_IDENTITY;
+use qiskit_circuit::gate_matrix::TWO_QUBIT_IDENTITY;
 use qiskit_circuit::imports::QI_OPERATOR;
 use qiskit_circuit::operations::{Operation, OperationRef};
 use qiskit_circuit::packed_instruction::PackedInstruction;
 use qiskit_circuit::Qubit;
 
-use crate::euler_one_qubit_decomposer::matmul_1q;
+use crate::qi::{VersorSU2, VersorU2};
 use crate::QiskitError;
 
 #[inline]
@@ -53,94 +52,147 @@ pub fn get_matrix_from_inst(py: Python, inst: &PackedInstruction) -> PyResult<Ar
     }
 }
 
+/// Quaternion-based collect of two parallel runs of 1q gates.
+#[derive(Clone, Debug)]
+struct Separable1q {
+    phase: f64,
+    qubits: [VersorSU2; 2],
+}
+impl Separable1q {
+    /// Construct an initial state from a single qubit operation.
+    #[inline]
+    fn from_qubit(n: usize, versor: VersorU2) -> Self {
+        let mut qubits: [VersorSU2; 2] = Default::default();
+        qubits[n] = versor.su2;
+        Self {
+            phase: versor.phase,
+            qubits,
+        }
+    }
+
+    /// Apply a new gate to one of the two qubits.
+    #[inline]
+    fn apply_on_qubit(&mut self, n: usize, versor: &VersorU2) {
+        self.phase += versor.phase;
+        self.qubits[n] = versor.su2 * self.qubits[n];
+    }
+
+    /// Construct the two-qubit gate matrix implied by these two runs.
+    #[inline]
+    fn matrix_into<'a>(&self, target: &'a mut [[Complex64; 4]; 4]) -> ArrayView2<'a, Complex64> {
+        let q0 = VersorU2 {
+            phase: self.phase,
+            su2: self.qubits[0],
+        }
+        .matrix_contiguous();
+
+        // This is the manually unrolled Kronecker product.
+        let [iz, iy, ix, scalar] = *self.qubits[1].0.quaternion().coords.as_ref();
+        let q1_row = [Complex64::new(scalar, iz), Complex64::new(iy, ix)];
+        for out_row in 0..2 {
+            for out_col in 0..4 {
+                target[out_row][out_col] = q1_row[(out_col & 2) >> 1] * q0[out_row][out_col & 1];
+            }
+        }
+        let q1_row = [Complex64::new(-iy, ix), Complex64::new(scalar, -iz)];
+        for out_row in 0..2 {
+            for out_col in 0..4 {
+                target[out_row + 2][out_col] =
+                    q1_row[(out_col & 2) >> 1] * q0[out_row][out_col & 1];
+            }
+        }
+        aview2(target)
+    }
+}
+
+/// Extract a versor representation of an arbitrary 1q DAG instruction.
+fn versor_from_1q_gate(py: Python, inst: &PackedInstruction) -> PyResult<VersorU2> {
+    let versor_result = if let Some(gate) = inst.standard_gate() {
+        VersorU2::from_standard(gate, inst.params_view())
+    } else {
+        VersorU2::from_ndarray(&get_matrix_from_inst(py, inst)?.view(), 1e-12)
+    };
+    versor_result.map_err(|err| QiskitError::new_err(err.to_string()))
+}
+
 /// Return the matrix Operator resulting from a block of Instructions.
+///
+/// # Panics
+///
+/// If any node in `op_list` is not a 1q or 2q gate.
 pub fn blocks_to_matrix(
     py: Python,
     dag: &DAGCircuit,
     op_list: &[NodeIndex],
     block_index_map: [Qubit; 2],
 ) -> PyResult<Array2<Complex64>> {
-    let map_bits = |bit: &Qubit| -> u8 {
-        if *bit == block_index_map[0] {
-            0
+    #[derive(Clone, Copy, PartialEq, Eq)]
+    enum Qarg {
+        Q0 = 0,
+        Q1 = 1,
+        Q01,
+        Q10,
+    }
+    let qarg_lookup = |qargs| {
+        let interned = dag.get_qargs(qargs);
+        if interned.len() == 1 {
+            if interned[0] == block_index_map[0] {
+                Qarg::Q0
+            } else {
+                Qarg::Q1
+            }
+        } else if interned.len() == 2 {
+            if interned[0] == block_index_map[0] {
+                Qarg::Q01
+            } else {
+                Qarg::Q10
+            }
         } else {
-            1
+            panic!("not a one- or two-qubit gate");
         }
     };
-    let mut qubit_0 = ONE_QUBIT_IDENTITY;
-    let mut qubit_1 = ONE_QUBIT_IDENTITY;
-    let mut one_qubit_components_modified = false;
+
+    let mut work: [[Complex64; 4]; 4] = Default::default();
+    let mut qubits_1q: Option<Separable1q> = None;
     let mut output_matrix: Option<Array2<Complex64>> = None;
     for node in op_list {
         let inst = dag[*node].unwrap_operation();
-        let op_matrix = get_matrix_from_inst(py, inst)?;
-        match dag
-            .get_qargs(inst.qubits)
-            .iter()
-            .map(map_bits)
-            .collect::<Vec<_>>()
-            .as_slice()
-        {
-            [0] => {
-                matmul_1q(&mut qubit_0, op_matrix);
-                one_qubit_components_modified = true;
-            }
-            [1] => {
-                matmul_1q(&mut qubit_1, op_matrix);
-                one_qubit_components_modified = true;
+        let qarg = qarg_lookup(inst.qubits);
+        match qarg {
+            Qarg::Q0 | Qarg::Q1 => {
+                let versor = versor_from_1q_gate(py, inst)?;
+                match qubits_1q.as_mut() {
+                    Some(sep) => sep.apply_on_qubit(qarg as usize, &versor),
+                    None => qubits_1q = Some(Separable1q::from_qubit(qarg as usize, versor)),
+                };
             }
-            [0, 1] => {
-                if one_qubit_components_modified {
-                    let one_qubits_combined = kron(&aview2(&qubit_1), &aview2(&qubit_0));
-                    output_matrix = Some(match output_matrix {
-                        None => op_matrix.dot(&one_qubits_combined),
-                        Some(current) => {
-                            let temp = one_qubits_combined.dot(&current);
-                            op_matrix.dot(&temp)
-                        }
-                    });
-                    qubit_0 = ONE_QUBIT_IDENTITY;
-                    qubit_1 = ONE_QUBIT_IDENTITY;
-                    one_qubit_components_modified = false;
-                } else {
-                    output_matrix = Some(match output_matrix {
-                        None => op_matrix,
-                        Some(current) => op_matrix.dot(&current),
-                    });
+            Qarg::Q01 | Qarg::Q10 => {
+                let mut matrix = get_matrix_from_inst(py, inst)?;
+                if qarg == Qarg::Q10 {
+                    change_basis_inplace(matrix.view_mut());
                 }
-            }
-            [1, 0] => {
-                let matrix = change_basis(op_matrix.view());
-                if one_qubit_components_modified {
-                    let one_qubits_combined = kron(&aview2(&qubit_1), &aview2(&qubit_0));
-                    output_matrix = Some(match output_matrix {
-                        None => matrix.dot(&one_qubits_combined),
-                        Some(current) => matrix.dot(&one_qubits_combined.dot(&current)),
-                    });
-                    qubit_0 = ONE_QUBIT_IDENTITY;
-                    qubit_1 = ONE_QUBIT_IDENTITY;
-                    one_qubit_components_modified = false;
-                } else {
-                    output_matrix = Some(match output_matrix {
-                        None => matrix,
-                        Some(current) => matrix.dot(&current),
-                    });
+                if let Some(sep) = qubits_1q.take() {
+                    matrix = matrix.dot(&sep.matrix_into(&mut work));
                 }
+                output_matrix = if let Some(state) = output_matrix {
+                    Some(matrix.dot(&state))
+                } else {
+                    Some(matrix)
+                };
             }
-            _ => unreachable!(),
         }
     }
-    Ok(match output_matrix {
-        Some(matrix) => {
-            if one_qubit_components_modified {
-                let one_qubits_combined = kron(&aview2(&qubit_1), &aview2(&qubit_0));
-                one_qubits_combined.dot(&matrix)
-            } else {
-                matrix
-            }
-        }
-        None => kron(&aview2(&qubit_1), &aview2(&qubit_0)),
-    })
+    match (
+        qubits_1q.map(|sep| sep.matrix_into(&mut work)),
+        output_matrix,
+    ) {
+        (Some(sep), Some(state)) => Ok(sep.dot(&state)),
+        (None, Some(state)) => Ok(state),
+        (Some(sep), None) => Ok(sep.to_owned()),
+        // This shouldn't actually ever trigger, because we expect blocks to be non-empty, but it's
+        // trivial to handle anyway.
+        (None, None) => Ok(arr2(&TWO_QUBIT_IDENTITY)),
+    }
 }
 
 /// Switches the order of qubits in a two qubit operation.
@@ -156,3 +208,14 @@ pub fn change_basis(matrix: ArrayView2<Complex64>) -> Array2<Complex64> {
     }
     trans_matrix
 }
+
+/// Change the qubit order of a 2q matrix in place.
+#[inline]
+pub fn change_basis_inplace(mut matrix: ArrayViewMut2<Complex64>) {
+    matrix.swap((0, 1), (0, 2));
+    matrix.swap((3, 1), (3, 2));
+    matrix.swap((1, 0), (2, 0));
+    matrix.swap((1, 3), (2, 3));
+    matrix.swap((1, 1), (2, 2));
+    matrix.swap((1, 2), (2, 1));
+}

crates/accelerate/src/lib.rs

@@ -38,6 +38,7 @@ pub mod isometry;
 pub mod nlayout;
 pub mod optimize_1q_gates;
 pub mod pauli_exp_val;
+pub mod qi;
 pub mod remove_diagonal_gates_before_measure;
 pub mod remove_identity_equiv;
 pub mod results;

crates/accelerate/src/qi/mod.rs

@@ -0,0 +1,18 @@
+// This code is part of Qiskit.
+//
+// (C) Copyright IBM 2024
+//
+// This code is licensed under the Apache License, Version 2.0. You may
+// obtain a copy of this license in the LICENSE.txt file in the root directory
+// of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+//
+// Any modifications or derivative works of this code must retain this
+// copyright notice, and modified files need to carry a notice indicating
+// that they have been altered from the originals.
+
+//! Quantum-information and linear-algebra related functionality, typically used as drivers for
+//! numeric compiler routines.
+
+mod versor_u2;
+
+pub use self::versor_u2::{VersorSU2, VersorU2, VersorU2Error};